1. Field of the Invention
The present invention relates to a zoom lens system for a compact camera, and to a focusing method thereof.
2. Description of the Related Art
As a zoom lens system for a camera, the following zoom lens systems are well-known in the art:
(i) a retrofocus type zoom lens system which includes a negative front lens group and a positive rear lens group in this order from the object, and in which zooming is performed by varying the distance therebetween; and
(ii) a telephoto type zoom lens system which includes a positive front lens group and a negative rear lens group in this order from the object, and in which zooming is performed by varying the distance therebetween.
The retrofocus type zoom lens system is mainly used in a single lens reflex (SLR) camera which requires a space for a mirror behind the photographing optical system.
The telephoto type zoom lens system is mainly used in a compact camera which does not require a long back focal distance.
Due to the two-lens-group arrangement of the above types, the number of lens elements is small, a structure of the zoom lens system can be simplified, and miniaturization thereof can be attained. However, if a zoom ratio becomes about 3 or more, it is difficult to correct field curvature occurred at any focal length along a zooming range defined by the short focal length extremity and the long focal length extremity.
In recent years, in addition to the requirement of miniaturization, a zoom lens system for a compact camera is required to have a high zoom ratio. In order to satisfy this requirement, in a compact camera with a telephoto type zoom lens system, a three-lens-group arrangement is frequently employed. In this zoom lens system, each of the three lens groups is independently moved along the optical axis upon zooming; and on the other hand, one of the first and second lens groups is moved along the optical axis upon focusing.
In order to attain miniaturization of a zoom lens system, it is important to reduce (i) the overall length of the lens system, (ii) the diameter thereof, and (iii) the thickness of each lens group. However, if lens elements are eliminated to reduce the thickness of a lens group, the number of lens elements constituting a focusing lens group is reduced accordingly. As a result, it becomes difficult to suitably correct aberrations from an infinite photographing distance to the closest photographing distance over the zooming range defined by the short focal length extremity and the long focal length extremity. This tendency becomes more noticeable when a zoom ratio is set higher. In order to correct aberrations suitably from infinity to the closest photographing distance, the number of lens elements in the focusing lens group has to be increased. Consequently, it is not practically possible to attain both miniaturization of a lens system and a high zoom ratio thereof.
It is an object of the present invention to provide a telephoto-type three-lens-group zoom lens system, with a small number of lens elements, which can suitably correct aberrations.
In the present invention, there is provided a zoom lens system which includes a positive first lens group, a positive second lens group, and a negative third lens group, in this order from the object, and in which each of the three lens groups is independently moved, along the optical axis, upon zooming so that the distances therebetween become shorter; and the zoom lens system satisfies the following condition:
0.03 less than (d12Wxe2x88x92d12T)/fW less than 0.15xe2x80x83xe2x80x83(1)
wherein
d12W designates the distance between the most image-side surface of the first lens group and the most object-side surface of the second lens group, at the short focal length extremity;
d12T designates the distance between the most image-side surface of the first lens group and the most object-side surface of the second lens group, at the long focal length extremity; and xe2x80x99fW designates the focal length of the entire zoom lens system at the short focal length extremity.
The first and second lens groups, which are independently moved upon zooming, are preferably moved together upon focusing. According to this arrangement, the number of lens elements constituting a focusing lens group can substantially be increased, and thereby the correcting of aberrations over the entire photographing range (i.e., from an infinite photographing distance to the closest photographing distance) can be made easily, and further no cost increase is incurred.
The zoom lens system preferably satisfies the following condition:
10 less than KF less than 20xe2x80x83xe2x80x83(2)
wherein
KF=(fT/fFT)2
fT designates the focal length of the entire zoom lens system at the long focal length extremity; and
fFT designates the resultant focal length, at the long focal length extremity, of the first lens group and the second lens group.
Further, the first lens group preferably includes three lens elements, and the second lens group preferably includes two lens elements, and thereby, with a small number of the lens elements, aberrations can suitably be corrected over the entire photographing range.
Still further, according to the present invention, there is provided a focusing method for a zoom lens system including a positive first lens group, a positive second lens group, and a negative third lens group, in this order from the object, wherein the method including the steps of:
(i) independently moving the first, second and third lens groups, along the optical axis, upon zooming so that he distances therebetween become shorter; and
(ii) integrally moving the first lens group and the second lens group along the optical axis upon focusing.
The present disclosure relates to subject matter contained in Japanese Patent Application No. Hei-11-144819 filed on May 25, 1999) which is expressly incorporated herein by reference in its entirety.